Recently, this investigator has obtained evidence that the L2 light chain of rabbit myosin enhances, 1) thin filament cooperativity (in press) and 2) actin interaction under conditions which are optimal for turnover of phosphorylated L2 (preliminary results). The present project will begin to utilize these observations to explain the low diastolic compliance of cardiac muscle in terms of a regulatory function for L2. Ca ions-sensitivity and relaxation will be studied as a function of the degree of phosphorylation of L2(L2P) in the skeletal and cardiac system. Preparation of "soluble" myosin will enable a kinetic analysis of the altered actomyosin complex. Soluble subfragments (HMM and/or Sl) will be prepared which either retain or can reassociate with L2 (and/or L2P) and the following parameters of interaction with pure actin will be studied, 1) the inhibitory effect of free Ca ion at low free Mg ion, 2) the inhibitory effect of free Mg ions and free ATP at 0.1 micronM Ca ions. Cosedimentation experiments will attempt to show that L2 binds to F-actin or to acto-L2S1. Experiments with cross-linking reagents will probe for the region of association of L2 with the heavy chain of the myosin molecule in an attempt to localize this light chain in the vicinity of the actin-binding site. The two classes of L2 will be labelled with fluorescent metal chelates in order to monitor changes in inter-"head" distances upon actin-binding. It is the aim of this project to begin to provide a molecular mechanism in terms of a regulatory function for L2, to explain cardiac performance as an equilibrium between MA.A-M.ADP.P complexes. The atheletic, hypertrophied or failing heart results from a shift from the normal equilibrium position.